Angle-resolved reflectometry is a technique that is used for measuring parameters such as overlay and critical dimension on a test pattern that is printed on a wafer. A cone of light is focused onto the test pattern and the reflectance of the pattern as a function of angle of incidence is collected in a pupil image. The exact intensity distribution of the light in the collected pupil relative to the intensity distribution in the illumination pupil provides the information necessary to extract precise measurements of the test pattern parameters.
As target sizes become smaller, due to cost and production constraints, applying angle-resolved reflectometry is becoming evermore challenging. The features that make up a test pattern are not uniform throughout the entire test pattern. They invariably contain imperfections such as edge roughness, variable line width and side wall slope inconsistencies. These imperfections in the test pattern features scatter or reflect light in a way that modifies the phase and intensity distribution of the light that is collected by the reflectometer.
Wafer optical metrology tools contain finite size apertures in field planes and in pupil planes. For example, FIG. 6 is a high level schematic illustration of a metrology optical system 90 according the prior art. FIG. 6 illustrates an optical fiber 91 as light source for inspection of a wafer 80 by a pupil image sensor 94. In this specific schematic example, light travels through an illumination pupil aperture 95A, an illumination filed aperture 95B, an objective pupil aperture 95C and a collection field aperture 95D.
These apertures may or may not be apodized and when placed in the plane reciprocal to the plane where the measurement is made, have the following simple purpose. Light arriving into the detector (be it a detector placed in field plane or a detector placed in pupil plane), contains signals from scattering and diffracting elements that are exterior to the metrology target, and that contaminate the sought for ideal signal. One of the aims of such aperture stops is to block this contaminating light. For example, when the signal is collected in pupil plane, a small aperture placed in a field plane on the collection arm, termed a collection field stop, blocks light from the exterior vicinity of the metrology target.
As the apertures become smaller, the filtering (done in field or in pupil plane) becomes more restrictive and the contaminating light mentioned above is removed in a more efficient way. This, however, comes at the cost of diffractions. Specifically, when the apertures size approaches the spatial coherence length of the radiation, the diffractions off the aperture edges modify the signal in a sizeable amount, and influence the metrology performance. Still, prior art methods suppress diffraction from the field stop by decreasing the spot size (achieved, for example, by means of pupil apodization done in the illumination) and/or choosing the field stop size or shape and a corresponding, judicially chosen region in the collection detector, so that the diffraction effect is small (this may be the case if the larger spot ringing happens on certain areas of the detector).